The most common approach to the control of the generation within each load distribution control area of an interconnected electric power system is that known as net interchange tie-line bias control which operates to control the output of the generators in each area so as to tend to maintain the area control error signal at zero when for each control area the area control error signal is calculated in accordance with the following equation: EQU ACE=(P.sub.t1 -P.sub.o)-10B(f.sub.m -f.sub.o) (1)
thus, EQU ACE=.DELTA.P.sub.t1 -10B(.DELTA.f) (2)
where,
ACE=the area control error, a positive control error indicating a need for reducing generation. PA1 P.sub.t1 =the measured net interchange of the area in megawatts. Power flow "out" of an area is considered as positive. PA1 P.sub.o =the scheduled net interchange of the area in megawatts, as preset. PA1 B=the frequency bias setting for the area in megawatts per 0.1 Hz., considered to have a minus sign. PA1 f.sub.m =measured system frequency in Hz. PA1 f.sub.o =system frequency schedule in Hz., as preset. PA1 net interchange schedule setting, P.sub.o, PA1 net interchange measurement, P.sub.t1, PA1 scheduled system frequency setting, f.sub.o, PA1 system frequency measurement=f.sub.m, and PA1 the rate of energy payback, 1/T
The control signals which effect the change in generation of the generators in each area are usually derived from the area control error with appropriate consideration for a number of other measured and computed parameters as necessary to optimize economy and security of the area and the system of which it is a part while always tending to reduce the area control error to zero.
When control action in each area is such that the area control error is reduced to zero, if the control operates in a hypothetically perfect manner, the interconnected areas which makes up the power system, if they all control on the same basis, will collectively achieve operation at the scheduled frequency and the scheduled interchanges. This hypothetically perfect operation assumes that the algebraic sum of all area net-interchange schedules is equal to zero and the common scheduled frequency f.sub.o is the same for all areas. Perfect control in each area is, of course, never fully realized. Thus, there are deviations from the scheduled frequency and the scheduled net interchange in each area resulting from the natural droop of the governors, metering errors and the delay in response of the control system in each area as well as the imperfections in that response. Those deviations from the scheduled net interchange create undesired but unavoidable energy interchanges between the areas. Those interchanges are known as inadvertent interchange and are quantitatively the time integral of the deviation of the areas net interchange from its net-interchange schedule. Thus, the inadvertent interchange, II, may be calculated in accordance with the following equation: EQU II=.intg.(P.sub.t1 -P.sub.o)dt (3)
thus, EQU II=.intg..DELTA.P.sub.t1 dt (4)
Inadvertent interchange includes two components. One is usually referred to as "intentional" inadvertent interchange. That component occurs when the area controls are effective and it results from the response of the governors on the areas generating capacity when the frequency is not at its scheduled value. Another component of the interchange is referred to as "unintentional". That component results from the failure of an area control system to reduce to zero the control error for the area.
Deviations from the scheduled frequency setting produce time deviations which must be corrected in order to maintain clocks reasonably close to correct time. The time deviation TD is quantitatively the time integral of the frequency deviation with an appropriate constant depending upon the magnitude of the scheduled frequency. Thus, time deviation in seconds accumulated in time t in hours may be calculated in accordance with the following equation: EQU TD=3600/f.sub.o .intg.(f.sub.m -f.sub.o)dt (5)
thus, EQU TD=3600/f.sub.o .intg..DELTA.fdt (6)
In controlling a large interconnection, it has become important to not only have each area maintain its scheduled net interchange and do its part in maintaining the scheduled frequency for the system, but also it is desirable to minimize the accumulated inadvertent interchange and time deviation.
Others have set forth methods for producing a modified area control error signal for the purpose of attempting to accomplish this task. One such system is disclosed in U.S. Pat. No. 3,898,442, issued Aug. 5, 1975, the disclosure of which is hereby incorporated by reference.
In the reference system, a modified area control error is obtained by adding two quantities or correction factors to the calculation of the normal area control error. A first one of those factors is obtained by periodically calculating from kilowatt-hour meter readings a quantity which is the total kilowatt-hour energy interchange, inadvertent interchange, between the area and the rest of the interconnected system for a predetermined period of time. That quantity is divided by a quantity representative of the period of time over which correction or payback of the energy interchange is desired. As suggested in the reference, the inadvertent interchange is calculated over a period of hours so that after each such period there is an updating of that quantity, identified in the reference as I.sub.a /H.
The other factor which the reference adds in determination of the modified area control error is a time error correction which is broadcast from a central point in the system. This factor is proportional to the frequency bias setting B for the area times the time integral over a particular time period of the frequency deviation, all divided by the time period during which it is desired to make the correction, which time period is the same as that over which the inadvertent interchange correction occurs.
The method of incorporating these factors in the modified area control error is all shown in FIG. 3 of the reference. The system there disclosed has several disadvantages in that the factors such as I.sub.a /H are not erased as corrective action is taken by the control system. Thus, with the prior art system, it is necessary to wait for a period of an hour or so before the control system can see the correction which it has accomplished. Also, contributions to the additional factors, such as I.sub.a /H, due to imperfect control must also wait the next updating of that quantity before the control system sees the corrective action required.
It is an object of this invention to provide a method of control and a means for carrying out that method with a modified area control error signal which will enable the control system of each area to minimize time deviation and inadvertent interchange in a manner which provides improved control in comparison with that available by systems known in the prior art.
It is a further object of this invention to provide for a control system including a modified area control error signal to provide control which makes up the deficiencies of the prior art systems as mentioned above so that the control continuously provides a corrective action tending to minimize inadvertent interchange and time deviation caused by imperfect control and measurement errors without having to wait for periodic updating of factors determinative of the corrective action required.